If you want to serve files greater than 64MB you'll need to set up or rent a web server and use it to store and serve files. This is known as a FastDL server.
Alternatively, you can use the Steam Workshop to serve your maps, if the maps are on the Workshop. Fortunately, Abandoned Upward is in the Workshop here: https://steamcommunity.com/sharedfil...andoned+upward
In order to run it on your server, you need the map's nine digit ID number which is found in its Workshop URL (454133929 in this case). In all cases where you want to switch to the map, whether it be manually or in your mapcycle.txt file, you type 'workshop/' followed by the map ID. For example:
Instead of downloading Workshop maps from your server, players download them from the Steam Workshop automatically.